1. Technical Field of the Invention
This invention relates to apparatus and processes for measuring physical parameters of sheet material by directing electromagnetic radiation from a source to the material and electronically processing infrared detector responses associated with selected wavelengths of radiation which passes through or otherwise interacts with the material.
2. Description of Prior Art
Measurement of physical parameters of sheet material through the use of infrared absorption phenomena is well-known. Typical procedures employ a source of electromagnetic radiation having a spectral output that includes the infrared region, a collimator or other arrangement for directing the radiation toward the sheet material, a chopper for modulating the directed radiation, filters for passing selected bands of radiation, one or more infrared detectors that produce responses which depend on the intensity of radiation passing through the filters, and electronic processing means for deriving the measurement from the detector responses. The filters may be mounted in a rotating chopper so that radiation transmissions are time-multiplexed to a single detector as in U.S. Pat. Nos. 4,052,615 Cho, or to separate, plural detectors as in 4,300,049 Sturm. Alternatively, the procedure may employ plural sources with two or more detectors as in U.S. Pat. No. 4,306,151 Chase, or a single source with a beam splitter and plural, separate detectors as in U.S. Pat. No. 3,405,268 Brunton.
The physical parameter in question is measured by taking advantage of the selective absorption of certain wavelengths of infrared radiation by certain constituents of the sheet material as taught, for instance, by U.S. Pat. No. 3,228,282 Barker. The typically heterogeneous nature of the sheet material introduces sources of measurement error, some of which can be compensated for by measuring the absorption for two or more different bands of radiation and interrelating the measurements to produce corrected measurements indicative of the physical parameter or parameters in question. This technique is exemplified in U.S. Pat. Nos. 3,405,268 Brunton and 4,577,104 Sturm.
Other sources of measurement error are compensated for by the geometric arrangement of the apparatus, as illustrated in U.S. Pat. Nos. 3,793,524 Howarth and 4,052,615 Cho.
Additional sources of error inhere in the methods by which certain components of such previous apparatus have been used. Specifically, those which use mobile filters to produce sequential detector responses in a time-multiplex arrangement may introduce sources of error as explained in U.S. Pat. No. 4,300,049 Sturm (cols. 3-4). In those which employ plural, separate detectors, the detectors are disposed on separate substrates and may be separately cooled. Either of these conditions may affect the relative thermal stability of the detectors and represent yet another source of error. Moreover, when plural detectors are used in conjunction with a beam splitter, the radiation emitted from the source is divided among the detectors, thereby yielding weaker detector responses. Where stronger responses are desired, the source intensity may be increased--which increases cooling requirements and decreases longevity for the source-- or the weak responses may be further amplified, which yields no improvement in signal-to-noise ratio and heightens electronic filtering requirements in conventional signal processing circuits.